With spread of the Internet, information on the Internet has increased explosively so that the user retrieves desired information from information existing on the Internet by using a search engine. In this case, the user conducts a search by entering a keyword related to desired information to the search engine. However, in the present circumstances, a search result varies largely depending on selection of a keyword, and the user cannot reach desired information immediately. The user bears a burden of selecting a keyword to efficiently reach desired information.
Consequently, a retrieving method enabling the user to perform a search only by entering a sentence without aware of a keyword is studied. For example, patent document 1 discloses a similar sentence retrieving program of performing a morphological analysis on an input sentence, determining a segment, analyzing dependency on the segment unit basis, arranging segments in appearance order, when a verb or a segment having no phase attachment appears, generating a compound word including the verb or segment and grouping all of segments before the verb or segment, recording the input sentence so as to be associated with the compound word into a sentence database, when an arbitrary original sentence is newly entered, generating a compound, and retrieving a sentence including a compound as a key from the sentence database using, as a key, the obtained compound word on the original sentence.